Bottom Line:
In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere.Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation.Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

ABSTRACTSelenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

nutrients-07-04199-f004: Schematic diagram of the biochemical reactions in the uptake and metabolism of Se in plants and microorganisms. Identified catalyzing enzymes and their Enzyme Commission (EC) numbers are indicated in orange at the corresponding reactions. Major (intermediate) compounds are indicated in bold. Information was compiled from previous reviews [11,34,49,55,117,185,193,194,195].

Mentions:
The uptake of Se and synthesis of Se amino acids has been investigated in algae [193,196] and plants [34,117]; a schematic overview is given in Figure 4. Briefly, intracellular selenate is reduced to selenite via activation with ATP-sulfurylase, and selenite may be further reduced to selenide, which may occur both enzymatically and non-enzymatically [34]. Subsequently, selenide is incorporated into SeCys via the coupling with O-acetylserine that is catalyzed by the enzyme cysteine synthase (Figure 4). In turn, SeCys may be further metabolized into SeMet via the methionine cycle, which includes enzymatic transformation of SeCys to Se-cysthationine, Se-homocysteine and finally SeMet [34]. The amino acids SeMet and SeCys are pivotal Se molecules because they have direct antioxidant functions and fulfill a number of essential physiological functions through deliberate incorporation of SeCys into Se-proteins [197,198,199]. On the other hand, non-specific incorporation of Se-amino acids (SeCys) proteins is a major cause of Se toxicity [200]. Furthermore, the Se-amino acids may be considered precursors of methylated, volatile Se species that can be emitted to the atmosphere. Biomethylation will be further discussed in chapter 7.

nutrients-07-04199-f004: Schematic diagram of the biochemical reactions in the uptake and metabolism of Se in plants and microorganisms. Identified catalyzing enzymes and their Enzyme Commission (EC) numbers are indicated in orange at the corresponding reactions. Major (intermediate) compounds are indicated in bold. Information was compiled from previous reviews [11,34,49,55,117,185,193,194,195].

Mentions:
The uptake of Se and synthesis of Se amino acids has been investigated in algae [193,196] and plants [34,117]; a schematic overview is given in Figure 4. Briefly, intracellular selenate is reduced to selenite via activation with ATP-sulfurylase, and selenite may be further reduced to selenide, which may occur both enzymatically and non-enzymatically [34]. Subsequently, selenide is incorporated into SeCys via the coupling with O-acetylserine that is catalyzed by the enzyme cysteine synthase (Figure 4). In turn, SeCys may be further metabolized into SeMet via the methionine cycle, which includes enzymatic transformation of SeCys to Se-cysthationine, Se-homocysteine and finally SeMet [34]. The amino acids SeMet and SeCys are pivotal Se molecules because they have direct antioxidant functions and fulfill a number of essential physiological functions through deliberate incorporation of SeCys into Se-proteins [197,198,199]. On the other hand, non-specific incorporation of Se-amino acids (SeCys) proteins is a major cause of Se toxicity [200]. Furthermore, the Se-amino acids may be considered precursors of methylated, volatile Se species that can be emitted to the atmosphere. Biomethylation will be further discussed in chapter 7.

Bottom Line:
In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere.Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation.Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

ABSTRACTSelenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.